Various techniques are known for dispersing hydrophobic photographically useful compounds such as photographic couplers into photographic element layer coating compositions comprising hydrophilic colloids.
Photographic dye forming couplers, as well as other hydrophobic photographically useful compounds, are typically incorporated into a hydrophilic colloid layer of a photographic element by first forming an aqueous dispersion of the couplers and then mixing such dispersion with the layer coating solution. An organic solvent is typically used to dissolve the coupler, and the resulting organic solution is then dispersed in an aqueous medium to form the aqueous dispersion.
The organic phase of these dispersions frequently includes high boiling or permanent organic solvents, either alone or with low boiling or water miscible solvents which are removed after dispersion formation. Permanent high boiling solvents have a boiling point sufficiently high, generally above 150.degree. C. at atmospheric pressure, such that they are not evaporated under normal dispersion making and photographic layer coating procedures. Permanent high boiling coupler solvents are primarily used in the conventional "oil-protection" dispersion method whereby the organic solvent remains in the dispersion, and thereby is incorporated into the emulsion layer coating solution and ultimately into the photographic element.
The conventional "oil in water" dispersion method for incorporating hydrophobic couplers is described, e.g., in U.S. Pat. No. 2,322,027 by Jelly and Vittum. In such conventional process, the coupler is dissolved in a high boiling water immiscible solvent, mixed with aqueous gelatin, and dispersed using a colloid mill or homogenizer. The presence of the high boiling solvent provides a stable environment for the hydrophobic coupler, as well as generally increasing the reactivity of the coupler upon photographic processing.
U.S. Pat. No. 2,801,170 of Vittum et al. discloses preparing separate dispersions of a coupler and a high boiling point solvent and mixing the two dispersions with a silver halide emulsion. U.S. Pat. No. 2,787,544 of Godowsky et al. discloses a method of making mixed packet photographic systems. A dispersion of high boiling point solvent is mixed with a dispersion of coupler. Both these processes help prevent crystallization of the coupler prior to layer coating by keeping the solvent and the coupler separate until just prior to coating, while providing solvent in the coated layer to enhance coupler reactivity in the photographic element.
While the presence of high boiling solvents in certain coupler dispersions in photographic elements is frequently desirable to provide, e.g., adequate coupler reactivity upon photographic processing, lower thermal yellowing, modified dye hues, enhanced dye dark stability, and reduced crystallization, there are also certain advantages which may result from the substantial elimination of high boiling permanent solvent from a photographic element imaging layer. U.S. Pat. No. 5,173,398, e.g., discloses photographic elements with coupler-containing layers having substantially no high boiling solvent. Alternative methods for delivering hydrophobic materials, such as color couplers, to aqueous based photographic compositions without using high boiling solvents are well known in the art. Such techniques involve, e.g., deposition of the couplers from basic aqueous solutions or volatile low boiling or water miscible organic auxiliary solvents followed by removal of the auxiliary solvent by evaporation or washing, mechanical dispersion methods wherein solid compounds are directly dispersed in an aqueous composition, and latex loading methods wherein, e.g., a coupler is loaded into a latex polymer by mixing a solution of the coupler in a low boiling or water miscible organic solvent with an aqueous suspension of the latex followed by removal of the solvent. Minimizing the amount of permanent coupler solvent coated in a photographic element is useful, e.g., for reducing the coated thickness of photographic layers. Reductions in coupler solvent level also afford concomitant reductions in gelatin level which leads to further reductions in coated dry thickness. Layer thinning is advantageous for reasons such as improved image sharpness due to reduced light scattering during exposure and increased developability due to shorter diffusion paths through the multilayer structure. This increase in developability can lead to lower silver and/or coupler coated levels, hence lower materials cost.
Additional advantageous properties may also be associated with the absence of coupler solvent in a particular layer. The light stability for yellow image dyes formed in an exposed and processed photographic element, e.g., may be significantly improved where the yellow dyes are formed in a photographic layer which is substantially free of organic solvents, such as disclosed in concurrently filed, commonly assigned, copending applications U.S. Ser. No. 08/916,882 (Kodak Docket No. 75629AJA) and U.S. Ser. No. 08/916,842 (Kodak Docket No. 76462AJA) the disclosures of which are incorporated by reference herein. It may be desirable to include organic solvents in cyan or magenta image dye forming layers while eliminating such solvent from the yellow dye forming layer, however, as coupler solvent reduction can frequently result in disadvantageous features in such layers such as reducing the reactivity of the dispersed cyan or magenta dye forming coupler to a level too low to produce desired dye density upon processing of the photographic material, higher thermal yellowing, lower dye dark stability, and increased crystallization. In balancing such requirements and desired features in a color photographic element, it thus may be desireable to include organic solvents in some image forming layers, while eliminating solvents from other image forming layers.
It has been found that the advantageous improvement in yellow image dye light stability associated with the removal of solvent from a yellow image forming layer of a photographic element may be degraded when an adjacent cyan or magenta imaging layer contains an organic solvent. It would be desirable to provide a silver halide photographic element which contains a yellow image forming layer which is substantially free of high boiling organic solvent and an adjacent cyan or magenta image forming layer which contains organic solvent in which the yellow image dye light stability is not significantly degraded by the presence of the solvent coated in an adjacent imaging layer.